The present invention relates to optical data recording media for high density recording, reproducing and erasing data with a laser beam and, more particularly, to optical data recording media, in which intercrystalline-amorphous structure and optical characteristics vary in dependence on the thermal hysteresis differences due to its temperature rise or fall caused by laser beam irradiation.
Optical disc recording systems using laser beam permit high capacity recording and contact-free fast accessing, and there is an increasing trend in their applications for high capacity memories. Optical discs are classified into those of reproduction or read only type known as compact discs and laser discs, those of write-once type capable of recording by users themselves, and re-writable type capable of repeated recording and erasing on the user side. Optical discs of the write-once and re-writable types are used as external memories for computers and document/image files.
Among re-writable optical discs are phase change type optical discs utilizing phase changes of recording films and magneto-optical discs utilizing changes in the magnetizing direction of perpendicularly magnetized films. The phase change type optical discs do not require any external magnetic field and readily permit over-writing, and are thus expected to become a main type of re-writable optical discs.
Commonly termed phase change type optical discs are well known in the art, which use recording films capable of undergoing inter-crystalline-amorphous phase changes in response to laser beam irradiation and are re-writable. In the phase change type optical disc, data is recorded as inter-crystalline-amorphous phase changes caused by locally elevating the recording film temperature with irradiation of the recording film by a laser beam spot of high power corresponding to the data to be recorded, and the recorded data is reproduced by reading optical constant changes accompanying it as reflected light intensity differences or phase changes with a low power laser beam.
For example, in a phase change optical disc using a recording film of a relatively long crystallization time, the recording film of the disc is elevated in temperature beyond the melting point in response to laser beam irradiation, and the irradiated film potions are made non-crystal by fast cooling them after the laser beam has passed, thereby effecting recording of data. When erasing data, the recording film is crystallized by holding the recording film temperature for a period of time sufficient for crystallization to proceed in a crystallizable temperature range above the crystallizing temperature and below the melting point. In a well-known method of doing so, a laser beam which is elongate in the direction of its process is used for irradiation. In the case of making two-beam psuedo over-writing for recording new data while erasing the data having been recorded, an oval laser beam for erasing is irradiated prior to the irradiation with the oval laser beam for recording.
In case of a disc using a recording film capable of being fast crystallized, a single circularly converged laser beam is used. In a well-know method, the laser beam power is changed between two levels for crystallization or non-crystallization. Specifically, when the recording film has been irradiated with a laser beam which can elevate the recording film temperature beyond the melting point, most of it becomes amorphous at the time of the cooling. On the other hand, portions of the recording film, which have been irradiated with a laser beam of such power that the recording film can reach a temperature above the crystallizing temperature and below the melting point, reach the crystal state.
The recording film of the phase change type disc is formed by using a charcogenite material, e.g., those of GeSbTe type, InSbTe type, InSe type, InTe type, AsTeGe type, TeO.sub.x --GeSn type, TeSeSn type, SbSeBi type, BiSeGe, etc. Using either material, the film is formed by a resistance heating vacuum deposition process, an electron beam vacuum deposition process, a sputtering process, etc. Right after its formation, the recording film is in a sort of amorphous state, and it is initialized to make it to be entirely crystalline for forming amorphous record portions by recording data on it. The recording is effected by forming amorphous portions of the film in the crystallized state thereof.
As conventional means for high density data recording on an optical disc, it is effective to combine mark edge recording and land/groove recording in combination.
Reflectance difference reproduction type media are well known in the art, are of a type for recording amorphous marks in a high reflectance crystalline portion and have a high reflectance difference between the crystalline and non-crystaline portions. In an application of such a reflectance difference reproduction medium for the mark edge recording, the absorptance of the crystalline portion is considerably lower than that of the amorphous portion because of absence of light transmitted through the medium. Therefore, recording mark distortion cannot be held low in high linear speed over-writing.
To overcome this drawback, a commonly termed phase difference reproduction type phase change optical disc has been proposed, in which the optical phase difference between crystalline and amorphous portions is reduced and the reflectance difference between the two portions is increased (as disclosed in, for instance, Japanese Laid-Open Patent Publication No. 7-93804). However, in this system it is necessary to set the phase difference to the neighborhood of 180 degrees. This means that it is necessary to accurately control the thicknesses of the individual layers of the medium so as to realize a desired phase difference.
FIG. 5 shows the structure of a prior art phase difference reproduction type medium. As shown, the structure is a laminate of substrate 1, first protective film 3, recording film 4, second protective film 5, reflective film 6 and protective region (or ultraviolet-setting resin) 7. The optical characteristic of this medium is shown in FIG. 6. As is seen from FIGS. 5 and 6, to realize the phase difference of 180 degrees which provides the utmost effects in the phase difference reproduction, the thickness of the first protective film may be set to the neighborhood of 45 or 195 nm. However, the thickness margins are narrow. From the standpoint of the thickness control in the film formation, the thickness of the first protective film is suitably 60 nm. With this structure, however, the substrate is thermally damaged by heating when recording data. Therefore, the structure is inadequate for repeated use.
In the land/groove recording as the other effective high density recording means, it is necessary to make even the amplitude levels of reproduced signals from groove and groove tracks of track guide grooves. To realize this, it is required to accurately set the intercrystalline-amorphous optical phase difference to the neighborhood of zero degree with the reflectance reproduction type medium and to the neighborhood of 180 degrees with the phase difference reproduction type medium. With the well-known medium having the structure as shown in FIG. 5, it is inevitable to set a large thickness of the first protective layer (to the neighborhood of 170 nm in the case of the first protective film in FIG. 6, for instance), and in this case the thickness variations for the first protective film greatly influence the phase difference.